Spinal disc herniation, a common ailment, often induces pain, as well as neurologically and physiologically debilitating processes, for which relief becomes paramount. If conservative treatments fail, the more drastic measures of discectomies and spinal fusion may be indicated. The latter treatment, while providing short term relief, limits spinal mobility and often leads to excessive forces on facet joints adjacent to the fusion and may create further problems over time. Drastic treatments are usually unable to restore normal disc function. The loss of disc function has led to a number of disc prostheses that attempt to provide natural motion.
The literature documents that the Instantaneous Axis of Rotation (IAR) during sagittal rotation of the superior vertebra with respect to the inferior vertebra of a Functional Spinal Unit (FSU) in the cervical spine moves significant distances during flexion and extension of the spine (Mameren H. van, Sanches H., Beursgens J., Drukker, J., “Cervical Spine Motion in the Sagittal Plane II: Position of Segmental Averaged Instantaneous Centers of Rotation—A Cineradiographic Study”, Spine 1992, Vol. 17, No. 5, pp. 467-474). This motion varies widely between functional spinal units on an individual spine and between individuals and can depend on age, time-of-day, and the general health and condition of the intervertebral discs, facet joints and other components of the FSU and spine. A moving IAR means that the superior vertebra can both rotate and translate while moving with respect to the inferior vertebra of an FSU. Natural spinal motions place severe requirements on the design of a prosthetic disc; simple rotational joints are not able to meet those requirements.
In addition, motion coupling between axial and lateral bending and other functional spinal units involved in the overall spinal motion increases the complexity and difficulty in developing a prosthetic disc replacement that realizes natural spinal motion. The complex facet surfaces in an FSU significantly influence and constrain sagittal, lateral and axial motions. The orientation of these facet surfaces vary with FSU location in the spine and induce wide variations in motion parameters and constraints. The complex motion of a superior vertebra with respect to the associated inferior vertebra of an FSU, certainly in the cervical spine, cannot be realized by a simple rotation or simple translation, or even a combination of rotation and translation along a fixed axis, and still maintain the integrity and stability of the FSU and facet joints.
Researchers have attempted to design a successful intervertebral disc for years. Salib et al., U.S. Pat. No. 5,258,031; Marnay, U.S. Pat. No. 5,314,477; Boyd et al., U.S. Pat. No. 5,425,773; Yuan et al., U.S. Pat. No. 5,676,701; and Larsen et al., U.S. Pat. No. 5,782,832 all use ball-and-socket arrangements fixed to the superior and inferior plates rigidly attached to the vertebrae of an FSU. However, these designs limit motion to rotation only about the socket when the two plates are in contact. As the literature points out (Bogduk N. and Mercer S., “Biomechanics of the cervical spine. I: Normal kinematics”, Clinical Biomechanics, Elsevier, 15 (2000) 633-648; and Mameren H. van, Sanches H., Beursgens J., Drukker, J., “Cervical Spine Motion in the Sagittal Plane II: Position of Segmental Averaged Instantaneous Centers of Rotation—A Cineradiographic Study”, Spine 1992, Vol. 17, No. 5, pp. 467-474), this restricted motion does not correspond to the natural motion of the vertebrae, either for sagittal plane motion or for combined sagittal, lateral and axial motion. Further, when the two plates, as described in the cited patents, are not in contact, the devices are unable to provide stability to the intervertebral interface, which can allow free motion and lead to disc related spondylolisthesis, FSU instability and excessive facet loading.
As a further elaboration on the many ball-and-socket configurations, consider Salib et. al. (U.S. Pat. No. 5,258,031) an example of previous efforts to address this problem. The Salib et al. ball-and-socket arrangement only provides 3 independent axes of rotation and no translation when engaged.
During complex motions of an FSU, the superior vertebra, in general, requires translation along three independent directions. A sliding ovate structure in an oversized socket cannot perform such general translation motions, either, as it must engage in a trajectory dictated by its socket's geometrical surface and does not change the deleterious effects that may occur on the facet joints of the unit.
Currently known devices appear to have similar motion and instability limitations, such as the rocker arm device disclosed by Cauthen (U.S. Pat. Nos. 6,019,792; 6,179,874; 7,270,681), the freely moving sliding disc cores found in the Bryan et al. patents (U.S. Pat. Nos. 5,674,296; 5,865,846; 6,001,130; and 6,156,067) and the SB Charité™ prosthesis, as described by Búttner-Jantz K., Hochschuler S. H., McAfee P. C. (Eds), The Artificial Disc, ISBN 3-540-41779-6 Springer-Verlag, Berlin Heidelberg New York, 2003; and U.S. Pat. No. 5,401,269; and Buettner-Jantz et al. U.S. Pat. No. 4,759,766). In addition, the sliding disc core devices of the Bryan et al. and SB Charité™ devices do not appear to permit natural motion of the joint for any fixed shape of the core.
With the above described prosthetic devices, when the FSU extends, the prosthesis's sliding core, in some cases, generates unnatural constraining forces on the FSU by restricting closure of the posterior intervertebral gap in the FSU. Further, the core does not mechanically link the upper and lower plates of the prosthesis and is unable to accommodate the changing intervertebral gap throughout the range of motion. Such conditions can contribute to prosthetic disc spondylolisthesis and/or transmission of large forces through the prosthesis not normally experienced with nominal loads. In general, unconstrained or over-constrained relative motion between the two plates in a prosthetic disc can contribute to FSU instability over time.
Various means of incorporating uniform, predictable, and kinematically restricted, relative lateral translation motion between plates without joint separation have been proposed, for example, by Zeegers in U.S. Pat. No. 7,695,516. Uniform, predictable two dimensional, and kinematically restricted, relative translational motion along a frontal axis from anterior to posterior and an orthogonal sagittal axis from left lateral to right lateral without joint separation has been proposed by Doty (U.S. Pat. Nos. 7,361,192; 7,799,080; 7,927,375 and U.S. Patent application US2010/0324688A1). Vertical translations relate closely with static and dynamic load handling prostheses and will be discussed next.
Current prosthetic disc technology appears to be limited in static and dynamic load handling capability. For example, load bearing and shock absorption in the SB Charité™ design and others (e.g. Bryan et al., U.S. Pat. No. 5,865,846) rely on the mechanical properties of the resilient, ultra-high-molecular-weight polyethylene core to provide both strength and static and dynamic loading. The rigidity of the sliding core appears to offer little energy absorption and flexibility to meet the changing intervertebral gap requirements during motion, and may generate excessive reaction forces on the spine during flexion, forces that can potentially produce extra stress on facet joints and affect mobility.
More recent attempts to provide dynamic and static loading capability is taught in the series of patents by Ralph et al. (U.S. Pat. Nos. 6,645,249; 6,863,688; 6,863,688; 7,014,658; 7,048,763; 7,122,055; 7,208,014; 7,261,739; 7,270,680; 7,314,487; and 7,713,302) wherein the force restoring mechanism begins with a multi-pronged domed spring between two plates followed by a wave-washer, then ending with a spiral grooved Belleville spring as the force restoring element. The multi-pronged domed spring employs a ball-and-socket arrangement on the upper plate and allows relative rotations between the spring-lower plate and the upper plate. This arrangement, during nominal FSU operation, places moments of force on the spring that tend to distort the spring and place high stresses on the set screws holding the spring down. The effects of force moments on the prongs and the dome spring is mitigated by later designs where various modifications of the spring element, as for example the spiral Belleville washer in U.S. Pat. No. 7,270,680, provides the spring more resilience to moments of force. As taught in these patents, the motion of the upper plate is limited to compression and rotation. Lateral and sagittal translations are not accommodated and so general motion in the FSU is not enabled by the device.
The work of Errico et al. (U.S. Pat. Nos. 6,989,032; 7,022,139; 7,044,969; 7,163,559; 7,186,268; 7,223,290; and 7,258,699) elaborates on the mechanical design of the patents of Ralph et al. A specially designed Belleville type washer provides a restoring force to compressions. Rotations of the superior plate of the device in a fixed ball-and-socket arrangement transfers moments of force about the washer central axis to a rigid structure. It is notable that the instruction in these designs specifically proscribes lateral motions (sagittal and lateral translation). Errico et al. employ a taper attached to the ball to limit rotation angles. De Villiers (U.S. Pat. No. 7,442,211) also describes how to control rotation angle limits using an annular joint stop on an ovate core. The rotational joint stop element taught here can apply non-uniform angle limits on rotations within a socket as well uniform rotation angle limits. It can be observed that in the De Villiers' device, rotations about the major axis of the ellipsoid are unrestricted.
Another approach to incorporate dynamic and static force response is taught by Gauchet (U.S. Pat. Nos. 6,395,032; 6,527,804; 6,579,320; 6,582,466; 6,582,468; and 6,733,532) wherein a hydraulic system provides shock absorption by means of a cushion between two plates contained within sealed flexible titanium bellows. Gauchet suggests the bellows can be designed to accommodate lateral forces and axial rotation that is permitted by the cushion, which, to allow sliding motion, is not attached to at least one plate. The titanium bellows can accommodate some axial rotations, but do not seem suitable for other rotations, which can cause excessive stresses on the bellows. A cushion internal to the cylinder, being flexible and not attached to at least one plate, can accommodate any rotation (U.S. Pat. Nos. 6,582,466 and 6,733,532).
Fleishman et al. in U.S. Pat. Nos. 6,375,682 and 6,981,989 utilize hydraulic action coupled with a flexible bellows to mitigate sudden forces. The bellows concept is similar to that of Gauchet.
Eberlein et al. (U.S. Pat. No. 6,626,943) utilize a fiber ring to enclose a flexible element. The forces and moments of force are absorbed by the ring and the flexible element. Other inventions teach a fiber ring type concept as well, namely, Casutt in U.S. Pat. No. 6,645,248. Diaz et al. (U.S. Pat. No. 7,195,644) also uses a membrane and enclosed cushioning material in their ball and dual socket joint design. Diaz also instructs that discontinuous segments of a connecting elastomer membrane around the periphery of a prosthesis can act as a plurality of elastic bands. The device taught in this invention can use a tough, fiber reinforced boot to provide a function similar to that of a fiber ring but with the added capability to seal in fluids/gases/gels within the device or seal out fluids and gases from interacting with the movable joints within the structure.
Middleton suggests a variety of machined springs as the central component of a disc prosthesis in U.S. Pat. Nos. 6,136,031; 6,296,664; 6,315,797; and 6,656,224. The spring is notched to allow static and dynamic response primarily in the axial direction of the spring. Lateral and sagittal translations and general rotations may be problematic in these designs. The ability of such springs to tolerate off-axis compression forces may also be problematic.
Gordon instructs deforming a machined spring as the principle separating and force management component (U.S. Pat. Nos. 6,579,321; 6,964,686; and 7,331,994). In U.S. Pat No. 7,316,714 the emphasis is on posterior insertion of a disc prosthesis that can provide appropriate motion. However, this latter design does not appear to accommodate static and dynamic loading and there appears to be no accommodation for lateral and sagittal translations.
Zubok instructs in U.S. Pat. No. 6,972,038 (Column 3; Line 35) that “ . . . the present invention contemplates that with regard to the cervical anatomy, a device that maintains a center of rotation, moving or otherwise, within the disc space is inappropriate and fails to properly support healthy motion.” This statement may be true as long as translations within the prosthesis mechanism do not adequately compensate for the total motion induced by an IAR outside of the disc space. The current invention, however, by sufficient means of three linearly independent translational degrees of freedom and three independent rotational degrees of freedom (for example, roll-pitch-yaw) within the FSU disc space, can generate any equivalent relative motion of the superior vertebra relative to an inferior vertebra within an FSU whose motion is generated by a moving IAR outside or inside the disc space. Further, the mechanisms of the current invention that generate the equivalent motion are so coupled that they can prevent separation of any moving elements of the prosthesis beyond mechanically programmed joint limits.
Several approaches by Ferree (U.S. Pat. Nos. 6,419,704; 6,706,068; 6,875,235; 7,048,764; 7,060,100; 7,201,774; 7,201,776; 7,235,102; 7,267,688; 7,291,171; and 7,338,525) primarily instruct how to cushion a prosthetic FSU in various ways. An exception is U.S. Pat. No. 6,706,068, which describes a design to perform certain kinematic motions of a disc prosthesis without dynamic or static cushioning support, and U.S. Pat. No. 7,338,525, which instructs on disc prosthesis anchoring.
Aebi incorporates essentially a hook joint (orthogonal revolute joints) in EP1572038B1 as the means for realizing motion. While the Aebi arrangement of revolute joints does allow for sagittal and lateral rotations, it does not appear to engage in the remaining four degrees of freedom in three-space, namely, sagittal, lateral, and axial translations along with axial rotations. Mitchell (U.S. Pat. No. 7,273,496B2) uses two revolute joints by means of orthogonal cylinders placed on top of each other and embedded as a crossbar element between plates with cavities for accepting the crossbar. This device has the limitations of motion similar to the Aebi device and the further limitation of not kinematically chaining the two plates together with the crossbar.
Khandkar (U.S. Pat. No. 6,994,727 B2) provides two orthogonal convex curvate bearing structures, with offset cylindrical radii of curvature, placed between the plates. An insert, with orthogonal, variable-curvature concave bearing surfaces, is placed between, and generally conforms to, the orthogonal convex bearings on the plates. This arrangement of bearings allows sagittal, lateral, and axial rotations of various ranges, dictated by the curvate bearing structures and the insert. The variable curvate surfaces allow some lateral and sagittal translations with FSU distractions, utilizing normal spinal forces to resist the distraction and, hence, the motion. There is no apparent control on the forces involved, so this method could lead to possible stress on other spinal joints. The inserted device is not kinematically chained to the rest of the device and can possibly be disengaged. Although, as instructed, the device is self-correcting within a limited range, tending towards a stable equilibrium established for the device in normal position. The variable curvatures result, typically, in line- and point-contact bearing manifolds that can wear the surfaces, possibly causing changes in the performance and characteristic motion of the device over time. In general, motion along the various manifold interfaces involved restricts the flexibility and adaptability of the device to accommodate other motions.
DiNello (U.S. Published Application No. 2006/0136062A1) instructs on how to adjust height and angulations of a motion disc after implantation.
Weber, in U.S. Pat. No. 7,582,115, introduces a prosthetic core which is a ball at one end and a plane at the other end that fits, respectively, into a curved socket superior plate and a plane surface inferior plate, allowing the core element to slide as well as permitting the superior plate to rotate about the ball end.
With respect to the lower vertebra in an FSU, all possible, natural loci of motion of any four non-planar, non-collinear points located in the superior vertebra define the natural workspace of an FSU. This workspace varies from one FSU to another on the spine and from one individual to another, creating considerable spinal disc prosthesis design problems.
The devices of the subject invention provide a general motion spatial mechanism. The device solves certain natural motion and shock absorbing characteristics that are problematic for a spinal disc prosthesis and offer a scalable mechanism for disc replacement without loss of general motion capabilities in the FSU.